This proposal is submitted to pursue our exploration of the pathogenesis of Parkinson's disease (PD). Pertinent to this goal, first, we have found, that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a toxin that kills dopaminergic (DA) neurons in the substantia nigra pars compacta (Snpc) as seen in PD, stimulates the production of hypochlorous acid, a highly reactive species that may contribute to MPTP's deleterious effects. To acquire a better understanding of hypochlorous acid cytotoxic role in the MPTP model, Specific Aim (SA)-1 will quantify brain levels of chlorotyrosine and nitrotyrosine, the two main fingerprints of hypochlorous acid-induced protein oxidative attack, at different time points and doses of MPTP. Since myeloperoxidase (MPO) is the only mammalian enzyme which produced hypochlorous acid, SA-II, will analyze the response of MPO mRNA and protein expression at different time points and doses of MPTP, and will assess the effects of MPTP on the Snpc DA neurons of knockout mice deficient in MPO. Second, we have demonstrated that MPTP can oxidatively damage proteins. However, MPTP can compromise other vital cellular elements, such as DNA, by a similar damage proteins. However, MPTP can compromise other vital cellular elements, such as DNA, by a similar process. However, MPTP can compromise other vital cellular elements, such as DNA, by a similar process. TO demonstrate whether DNA is an intracellular target of MPTP-induced oxidative damage and to explore the consequences of such alteration, SA-III will study the occurrence of DNA strand breakage and of poly(ADP-ribose) polymerase (PARP) activation in the Snpc after MPTP administration. Third, we have found that transgenic mice expressing mutant copper/zinc superoxide dismutase (mSOD1) not only exhibit a dramatic loss of spinal cord motor neurons, but also of Snpc DA neurons. This raises the unique possibility that mSOD1 produces a model of progressive adult-onset degeneration of Snpc DA neurons. To further investigate this model, SA-IV will characterize the neuropathology of mSOD1-mediated Snpc DA neuron sensitivity to MPTP. This proposal contains a comprehensive set of experiments, which would provide insight into the role of hypochlorous acid and its synthesizing enzyme MPO, as well as into DNA damage and PARP activation in the MPTP model. It should also generate significant information on the mSOD1 model of Snpc neurodegeneration. Collectively, the proposed studies should shed light on the mechanisms of neurodegeneration in PD.